Perhaps your bottleneck is not related to video processing.

You say you are running a Q6600, have you overclocked it to at least 3.0ghz? Have you set your different instances to different processor affinities? WoW will properly utilize two cores, so setting instance 1&2 to CPU#0/1 and instance 3,4&5 to CPU#2/3 would definitely be better than letting the application decide (which will cram everything onto CPU#0/1)

8gb RAM + WoW = TehSuck
WoW will only utilize the first 4gb because it is a 32bit application. Using 1gb+ of this on video memory will only compound this problem, because now you only have 3gb available to system memory. I'm sure you are using virtual memory (the old pagefile.sys) Maybe that read/write time is contributing to your low performance. There is no real solution for this afaik except for Raid0 Raptors

FPS caps. I set my slaves to cap at 15fps. I don't really play from their screens, but only monitor them, so perfect animation is not as pertinent as it is for the main box. This should conserve power for your focus, maybe.